[1] Polyesters and Polyester Films
In recent years, there is a rapid increase in demand for flat panel displays such as liquid crystal displays and plasma displays which are used as a display for personal computers, televisions, cellular phones, personal digital assistants, car navigation systems, liquid crystal projectors and clocks.
The flat panel displays are constructed of various optical films such as a polarizing plate, a phase difference film, a prism sheet and an anti-reflection film. Among properties required for these films, a birefringence contributing to an image-forming property is one of important optical properties thereof. In general, the optical films presently used are classified into optically isotropic films having a small birefringence such as protective films for polarizing plates, and optically anisotropic films exhibiting a birefringence to some extent such as phase difference films. The optically anisotropic films are produced by subjecting the optically isotropic films to stretching, etc. From this viewpoint, the optically isotropic films are more important.
As the optically isotropic films, there are known films made of triacetylcellulose, polycarbonates, amorphous cyclic polyolefins, polyether sulfones, polyarylates and polyesters. Almost all of these films are produced by a casting method in which a solution of a resin cast and formed into a film while evaporating the solvent (Patent Documents 1-6). However, the casting method tends to be considerably deteriorated in productivity, and has problems such as adverse influence owing to a residual solvent in the films. Further, the casting method as a production method using a solvent is also undesirable from the standpoint of reducing burdens on environments.
To solve the problems, there have been proposed other film forming methods using a melt-extrusion method. For example, there has been proposed a method of obtaining optically isotropic films by melt-extruding amorphous cyclic polyolefins (Patent Document 7). However, the resultant films is expensive because expensive amorphous cyclic polyolefins are used, and the bonding between the film and another material needs a special adhesive because of a low polarity of the amorphous cyclic polyolefins.
There has been also proposed a method of forming polyether sulfones into a film by a melt-extrusion method (Patent Document 8). However, the obtained films are also expensive because expensive raw resins are used, a smooth surface is difficult to obtain, and the film forming process is complicated.
There has been further proposed a method for obtaining optically isotropic films by reheating a polycarbonate film produced by an extrusion method (Patent Document 9). However, the method is economically disadvantageous because the increased number of steps are required, and the surface of the film is likely to be damaged during the film forming process. Thus, the conventional extrusion-molding methods have failed to produce optically isotropic films in an economically suitable manner.
[2] Phase Difference Films
The phase difference films are produced by stretching an optically isotropic film to allow the film to exhibit a birefringence. The phase difference films are an important member for image display devices such as liquid crystal displays to enhance the contrast and broaden the viewing angle by the optical compensation. Examples of resins generally used for forming the phase difference films include engineering plastic resins such as polycarbonates (PC), triacetylcellulose (TAC) and cycloolefin polymers (COP). The phase difference films are produced by forming these resins into a raw film by a casting method or a melt-extrusion method and then stretching the obtained raw film so as to exhibit a desired retardation.
The raw film having an optional thickness is required to have a small thickness unevenness, a low retardation, a small retardation unevenness and a high ability of generating a retardation even when stretched at a low stretch ratio. By stretching a raw film having these properties, phase difference films having a less unevenness in the thickness and optical properties are obtained.
Also, in recent years, with the entry of foreign competitors into the Japanese market and development of new type displays other than liquid crystal displays such as SED (surface-conduction electron-emitter display), makers of displays have now made severe price-cutting competition for survival. Under these circumstances, the makers of electric and electronic equipments have been forced to reduce the costs of liquid crystal displays, this requiring to bring the costs of various parts down. Therefore, there is a demand for further reducing the costs of phase difference films which are indispensable to liquid crystal displays.
However, in general, the raw PC or TAC films have been conventionally produced only by a casting method requiring high production costs, because a low-cost melt-extrusion method hardly produces films having a low retardation. In addition, COP films produced by the conventionally known melt-extrusion method tend to generate a retardation because of the orientation of polymer chains or stress thereon during the molding (Patent Document 10). Further, since optimum production conditions are unstable, there tends to arise such a problem that a very long period of time is required until reaching a stable production of the raw film sheets satisfying properties as required, resulting in poor yield and high production costs.
In order to prevent the discoloration and the deterioration in contrast of display colors due to a birefringence of STN (super twisted nematic) liquid crystal cell, there has been proposed a method of superimposing an inverse-twisted STN liquid crystal cell thereon (phase compensation of STN liquid crystal cell) (Non-Patent Document 1). Since the liquid crystal has a continuously oriented structure, a birefringence of the liquid crystal must be compensated by using a STN liquid crystal cell also having a continuously oriented structure. Also, in Non-Patent Document 1, a plurality of phase difference films are laminated on the liquid crystal cell in place of the inverse-twisted STN liquid crystal cell, and it is described that the same effect as that of the inverse-twisted STN liquid crystal cell is obtained by laminating 10 phase difference films such that the slow axes thereof are offset from each other. It is also described that although even one phase difference film exhibits the effect to a certain extent, the lamination of two phase difference films is more effective. However, the method of laminating the phase difference films is not described.
In addition, in Non-Patent Document 2, there is disclosed a method of laminating a phase difference film (referred to as an optical compensation film in Non-Patent Document 2) between a polarizing plate and a glass substrate. Since a polarizing plate made of only a polyvinyl alcohol (PVA) stretched film has a poor strength and tends to undergo a considerable change in dimension and shape upon exposure to heat or moisture, a triacetylcellulose (TAC) layer as a protective layer is generally laminated on both surfaces of the polarizing plate (Non-Patent Document 2).
[3] Protective Film for Polarizing Plates
The protective film for polarizing plates is obtained by using an optically isotropic film.
The liquid crystal displays have come to be widely used year by year as a space-saving image displaying device with low power consumption. The conventional liquid crystal displays have such a large disadvantage that images largely depend on a viewing angle. However, in recent years, a wide viewing angle liquid crystal mode such as VA mode and IPS mode has been developed and put into practice, this rapidly increasing the demand of the liquid crystal displays in the applications requiring a wide viewing angle such as televisions. The liquid crystal displays are constituted from liquid crystal cell, orientation film, polarizing plate, phase difference film, viewing angle-expansion film and backlight. The polarizing plate used in the liquid crystal displays is also required to be further improved in quality and productivity.
Typical optical films used in the liquid crystal displays include a protective film for polarizing plates, an orientation film, a phase difference film, a viewing angle-expansion film, etc. The orientation film is directly contacted with liquid crystal and has a function of orienting the liquid crystal relative to a substrate. Examples of a typical material of the orientation film include aromatic polyimides. The phase difference film is used for optical compensation, and serves for preventing the occurrence of viewing angle dependency such as the optical strain due to birefringence and the discoloration of displayed colors due to modulation in a direction of the viewing angle. Examples of a typical material of the phase difference film include polycarbonates and triacetylcellulose (TAC). In addition, in recent years, bulky cyclic olefin resins such as “Zeonor” available from Zeon Corporation and “Arton” available from JSR Corporation have also been used. The viewing angle-expansion film serves for producing a clear image even when viewed from the inclined direction. Examples of a typical material of the viewing angle-expansion film include stretched TAC films and films composed of a film substrate and an oriented discotic liquid crystal applied on its surface.
The polarizing plate is capable of allowing only a light polarized in a specific direction of a random polarized light (non-polarized light) such as natural light to pass therethrough, and is usually constituted from a polarizing film and a protective film. The polarizing film is formed from a polyvinyl alcohol-based stretched film dyed with iodine or a dichromatic dye. The protective film is a transparent resin film provided on one or both surfaces of the polarizing film for the purpose of protecting the polarizing film, and is required to have an optical transparency, a uniform thickness, a low retardation which is expressed by a product of birefringence and thickness, a small retardation unevenness, and a low moisture absorption. If the in-plane retardation is large, the retardation unevenness is high or the thickness unevenness is high, the image quality of liquid crystal displays is considerably deteriorated. Namely, the color irregularity phenomenon in which displayed colors are partially faded and the deflection of images occurs. At present, as the protective film for polarizing plates, there have been most extensively used TAC films having a good transparency, a low birefringence and an adequate rigidity (Non-Patent Document 3).
During the production of these films, the films being produced undergo various stresses due to melt-flowing of resins, drying contraction upon removal of solvents, heat shrinkage, stress upon transportation, etc. Therefore, there tends to occur such a problem that the resultant films have a residual retardation due to birefringence which is attributed to the molecular orientation induced by these stresses. These films have been generally produced by a solution casting method or a melt-extrusion method. The optical films such as the above protective film for polarizing plates are required to exhibit not only good optical properties with an extremely high accuracy, but also a uniform thickness and a good appearance as especially important properties thereof. Therefore, the films have been produced by using the solution casting method. More specifically, the protective film for polarizing plates has been produced by subjecting a concentrated solution of TAC to filtration, casting the filtrate on an endless support such as a roll and a band to form a self-supporting film, and then separating the film from the support, followed by removing the solvent by drying.
However, the solution casting method tends to be deteriorated in productivity and requires high production costs as compared with the melt-extrusion method because the former method needs the solvent removal step. If the time for the removal of the solvent is shortened to avoid these problems, there tend to occur other drawbacks such as the whitening of the films and the increase in retardation thereof and its unevenness, thereby making the production of the protective film for polarizing plates having desired properties difficult. Also, the complete removal of the solvent from the film is difficult. If the residual solvent is prevent in the film, the film undergoes stress unevenly upon stretching, thereby failing to realize a uniform retardation. When such a film is applied to liquid crystal displays such as portable OA devices and displays for automobiles which are used under conditions where temperature changes largely, the film tends to suffer from warpage, resulting in poor image quality. Also, the complete removal of the solvent needs an expensive drying apparatus to increase the facility costs and consumes a large quantity of energy to increase the running costs. Since a large amount of an organic solvent such as methylene chloride is used upon production of the film, there tend to arise additional problems such as adverse influence on heath of workers and environmental pollution due to volatilization of the solvent into atmospheric air.
Under these circumstances, in recent years, it has been attempted to use the melt-extrusion method in place of the solution casting method for producing the optical films. For example, there has been proposed a method for producing an optical polycarbonate film exhibiting a low in-plane retardation (10 nm or lower) in a visible range by the melt-extrusion method (Patent Document 11). However, since the in-plane retardation of the film obtained by the melt-extrusion method is still as high as from 22 to 50 nm, the film produced by this method should be held in a heating apparatus such as an oven and a drying furnace for a predetermined time while applying a tension in the processing direction to reduce the retardation to 10 nm or lower. Thus, since the film having a low in-plane retardation is not produced only by the melt-extrusion method, a low retardation is achieved by the subsequent heat-treatment step.
In addition, the water vapor permeability of the TAC films tends to be too high to use the films as a protective film for polarizing plates. If the protective film for polarizing plates has a high water vapor permeability, the wet heat resistance is reduced, the polarizing performance is reduced due to the dissociation or elimination of iodine in the polarizing film, and the warpage of the polarizing plate may occur. To solve these problems, there have been proposed various techniques for preventing the films from suffering from deterioration in wet heat resistance. However, many of these techniques relate to the methods of reducing a water vapor permeability by adding a hydrophobic additive to TAC or introducing a hydrophobic group into TAC (Patent Documents 12-15). However, TAC films excessively hydrophobilized tend to be hardly bonded to the polarizing film. In addition, some of the additives tend to cause the films to exhibit a birefringence, resulting in a high retardation of the obtained films. Thus, conventionally, it has been quite difficult to obtain the protective film for polarizing plates which satisfies all of a high yield, good optical properties such as a low retardation and a high total light transmittance, a low water vapor permeability and a suitable adhesion to the polarizing film at the same time.
[4] Lens Sheet
The lens sheet is produced by using an optical isotropic film as a substrate.
In recent years, color liquid crystal displays have been extensively used in various application fields such as liquid crystal monitors for portable note-type personal computers and disk top-type personal computers, monitors for liquid crystal televisions and car navigation systems, and monitors for cellular phones. Since the liquid crystal itself is not a self-light emitting element, a device called a backlight is used for lighting the liquid crystal from the back side. The backlight is constituted from a fluorescent tube, a light guide plate, a reflection sheet, a prism sheet, etc. The prism sheet is disposed on the light emitting surface of the light guide plate and serves for improving the optical efficiency of the backlight to enhance the luminance. For example, the prism sheet is obtained by arranging prisms having a triangular sectional shape on a resin film in parallel rows to form an optical element thereon. Further, there may be also used a lens sheet provided with an optical element having Fresnel lenses concentrically arranged on the surface of the resin film (Fresnel lens sheet). In addition, there may be also used a lens sheet provided with an optical element having lenticular lenses formed by arranging a plurality of cylindrical lenses in parallel rows on a surface of the resin film (lenticular lens sheet). The prism sheet, the Fresnel lens sheet and the lenticular lens sheet are generally referred to as lens sheet.
The prism sheet is generally produced by filling a mold having a desired prism pattern with an activation energy radiation-curable resin, superimposing a transparent substrate on the resin and irradiating the resin with an activation energy radiation through the transparent substrate to cure the resin. As the transparent substrate, there may be frequently used a stretched heat-set polyethylene terephthalate film (O-PET) from the standpoints of a good mechanical strength, low costs and a good transparency (Patent Document 16). However, in order to prevent the heat shrinkage due to the irradiation heat upon curing, the amount of radiation energy must be reduced, thereby preventing the improvement of the productivity. In addition, the process for production of the O-PET includes various steps such as melt-extrusion, stretching and heat-setting and, therefore, is complicated (Patent Document 17). Further, in the applications, particularly those exposed to high temperature such as monitors for car navigation systems and cellular phones, the O-PET is required to have a large thickness from the standpoint of a good dimensional stability, thereby preventing the reduction in the thickness. Also, as described in Non-Patent Document 4, since molecular orientation is undesirable for optical films, there is a demand for resin films which are non-oriented (low retardation) and show a good strength without stretching. However, no resin films satisfying these requirements have been presently obtained.
[5] Light Diffusion Film
The light diffusion film is produced by using an optical isotropic film as the substrate.
Conventionally, polyethylene terephthalate stretched films (PET stretched films) have been used as the substrate of the light diffusion films for use in liquid crystal displays because of their excellent mechanical strength, heat resistance and dimensional stability at high temperatures.
In recent years, to improve the contrast of liquid crystal display panels and increase their size, a light source of the backlight is required to emit an increased amount of light. Although the conventional PET films are improved in heat resistance by stretching, the heat resistance is still insufficient, thereby failing to emit an increased amount of light because of the temperature rise during use. To solve the problem, there has been proposed a film for light diffusion plates which is obtained by biaxially stretching a raw film made of polyimide and polyethylene terephthalate to improve the heat resistance (Patent Document 18). However, the stretching requires high costs similarly to the conventional PET stretched films. Further, as described in Non-Patent Document 4, November 4, in order to avoid the undesirable molecular orientation, there is a demand for resin films which are non-oriented (low retardation) and show a good strength without stretching. However, no resin films satisfying these requirements have been presently obtained.
[6] Anti-Reflection Film
The anti-reflection film is produced by using an optical isotropic film as the substrate.
In recent years, there is a rapid increase in demand for flat panel displays such as liquid crystal displays, plasma displays and projection displays which are used as an image display device for personal computers, televisions, cellular phones, personal digital assistants, car navigation systems, liquid crystal projectors and clocks. In these image display devices, in order to suppress deterioration in visibility due to reflection of an external light, the image display devices are provided with an anti-reflection film as an outermost layer.
The anti-reflection film is composed of a substrate mainly made of triacetylcellulose (TAC) or polyethylene terephthalate (PET), a hard coat layer and an anti-reflection layer each being laminated on the substrate.
The TAC substrate is produced by a solution casting method in which a solution is prepared by dissolving TAC having a bonded acetic acid amount (acetylation degree) of 60 to 62% together with a plasticizer in a mixed solvent composed of methylene chloride and methanol, the solution is continuously cast, and then the solvent is evaporated from the cast solution. However, the solution casting method requires a prolonged time and a large amount of energy for the dissolving step and drying step, resulting in high costs and environmental problems (Patent Document 19). Since the TAC film is easily broken during the coating of the anti-reflection layer, a continuous take-up coating method is hardly applicable. Therefore, the coating should be conducted by a single coating manner, resulting in poor productivity (Non-Patent Document 5).
Since the PET unstretched films are poor in the heat resistance, they may heat-shrink when coating the film with an anti-reflection layer by vapor deposition. Therefore, the amount of energy radiation irradiated on the films must be reduced, this being a cause for preventing the improvement of productivity. Further, the process for producing PET films which are enhanced in heat resistance by stretching and heat-setting is complicated because of various steps such as melt-extrusion, stretching and heat-setting (Patent Document 20). In addition, in the application, particularly those exposed to high temperatures such as monitors for car navigation systems and cellular phones, the stretched PET films are required to have a large thickness from the standpoint of a good dimensional stability, thereby inhibiting the reduction of thickness. Also, as described in Non-Patent Document 4, since molecular orientation is undesirable for optical films, there is a demand for resin films which are non-oriented (low retardation) and show a good strength without stretching. However, no resin films satisfying these requirements have been presently obtained.
[7] Optical Information Recording Medium
The optical information recording medium is produced by using an optical isotropic film as the protective layer.
In recent years, development of high-density optical information recording rapidly proceeds. For example, ultra-high density optical disks such as blu-ray discs have now been put into practice. The blu-ray discs having a diameter of 120 mm are capable of recording a large capacity data of 23 GB (giga byte) or more by a single-layer recording, and 47 GB or more by a dual-layer recording. In the blu-ray discs, the high-density recording is realized by reducing the track pitch of grooves on the disks to about 0.32 μm by using an optical system having a recording/reproducing wavelength of about 405 nm and a numerical aperture of about 0.85. With such a large numerical aperture, the distance between a pick-up lens and an information recording/reproducing layer (also merely recording layer) is very small as compared with current DVD disks. Therefore, the protective layer for the recording layer is required to have a thickness as extremely small as 100 μm. In addition, since the information is reproduced by using a polarized laser, the protective layer is required to be optically isotropic. The protective layer is usually constituted from a transparent adhesive layer and an optically isotropic film, and it is strongly required to produce the optically isotropic film at low costs.
In the standard of the blu-ray discs, the thickness of the protective layer formed on a laser incident side of the information recording/reproducing layer is limited to 100 μm (±2 μm). The optical properties required for the protective layer include an in-plane retardation of 5 nm or less at a wavelength of 405 nm. The protective layer is usually mainly made of a film of polycarbonate (PC). However, the PC film has a poor productivity, resulting in high production costs of the disks. Also, it is well known in the art that the PC film produced by melt extrusion hardly shows a reduced retardation. Therefore, the PC film fails to satisfy the properties required as an optically isotropic film for forming the protective layer of the blu-ray discs (Non-Patent Document 6).
The blu-ray films are usually constituted from a substrate having guide grooves, a reflection layer and a recording layer mainly composed of an organic pigment which are sequentially formed on the substrate, and a protective layer formed on the recording layer (Patent Document 21).    Patent Document 1: JP 9-95544A    Patent Document 2: JP 7-256664A    Patent Document 3: JP Patent 3404027    Patent Document 4: JP 7-73876A    Patent Document 5: JP 8-318538A    Patent Document 6: JP 7-41572A    Patent Document 7: JP 2003-279741A    Patent Document 8: JP Patent 3035204    Patent Document 9: JP Patent 2769020    Patent Document 10: JP 2004-109355A    Patent Document 11: JP 2003-302522A    Patent Document 12: JP 2002-22956A    Patent Document 13: JP 2002-146044A    Patent Document 14: JP 2001-343528A    Patent Document 15: JP 9-90101A    Patent Document 16: JP 10-197702A    Patent Document 17: JP 2004-131728A    Patent Document 18: JP 2002-341114A    Patent Document 19: JP 7-11055A    Patent Document 20: JP 2004-131728A    Patent Document 21: JP 2005-186607A    Non-Patent Document 1: Kobayashi, Hirakata and Nagae “Analysis of Phase Plate type Monochrome STN-LCD”, Singaku Technical Report, Vol. 88, No. 54, pp. 9 to 16, 1988    Non-Patent Document 2: Satake “Adhesives for Polarizing Plates”; Bonding Technologies, Vol. 25, No. 1, 2005, No. 78, pp. 25 to 30    Non-Patent Document 3: Optical films for Displays” edited by Fumio Ide, CMC Publishing, 2004    Non-Patent Document 4: “Analysis of Demand and Competition for Optical Transparent Plastic Films”, Fuji Chimera Research Institute, Inc., Nov. 4, 2004, p. 128    Non-Patent Document 5: “Prospect and Strategy of High-Performance Film Market 2005”, published by Yano Research Institute Ltd., pp. 86 to 87    Non-Patent Document 6: Yahata, Kazuo “Features and Mold Processing Techniques of Optical Transparent Resins, and Application to Optical Films/Mold Processing Techniques and Development of Optical Applications of Polycarbonate Films”, Technology Information Institute, Mar. 28, 2005, pp. 1 to 36